If a continuous metallic earth conductor
exists from the star point of the supply transformer to
the earthing terminal of the installation, it will run throughout
in parallel with the installation neutral, which will be
at the same potential. It therefore seems logical that one
of these conductors should be removed, with that remaining
acting as a combined protective and neutral conductor (PEN).
When this is done, we have a TN-C-S installation {5.2.4}.
The combined earth and neutral system will apply only to
the supply, and not to the installation.

Because of possible dangers with the system
which will be explained in the following sub-sections, PME
can be installed by the Electricity Supply Company only
after the supply system and the installations it feeds have
complied with certain requirements. These special needs
will be outlined in {5.6.4}.

The great virtue of the PME system is that
neutral is bonded to earth so that a phase to earth fault
is automatically a phase to neutral fault. The earth-fault
loop impedance will then be low, resulting in a high value
of fault current which will operate the protective device
quickly. It must be stressed that the neutral and earth
conductors are kept quite separate within the installation:
the main earthing terminal is bonded to the incoming combined
earth and neutral conductor by the Electricity Supply Company.
The difficulty of ensuring that bonding requirements are
met on construction sites means that PME supplies must not
be used. Electricity Supply Regulations forbid the use of
PME supplies to feed caravans and caravan sites.